1. Field of the Invention
The present invention relates to nonvolatile memory such as nonvolatile random-access memory (RAM), whose data are not lost irrespective of power-off events and which can be accessed in a random manner.
This application claims priority on Japanese Patent Application No. 2007-42230, the content of which is incorporated herein by reference.
2. Description of the Related Art
Conventionally, various types of nonvolatile memory such as flash memory (e.g., read-only memory (ROM) reloadable by user on board) have been known. For example, Japanese Unexamined Patent Application Publication No. 2001-14871 teaches a flash memory having security and protection functions.
In general, in the computer system adapted to cellular phones as shown in FIGS. 10 and 11, program codes are stored on hard disks or reloadable ROM such as flash memory in connection with processors, wherein program codes are loaded into and executed by volatile memory such as static random-access memory (SRAM) and dynamic random-access memory (DRAM).
The aforementioned computer system needs multiple storage media, which in turn increase the size of electronic devices; hence, it is very difficult to apply them to portable small-size electronic devices such as cellular phones.
It may be effective for portable small-size electronic devices to incorporate the system in which program codes are stored in reloadable ROM such as electronically erasable and programmable read-only memory (EEPROM) and flash memory so as to perform processing in accordance with programs, wherein nonvolatile memory such as static random-access memory (SRAM) and dynamic random-access memory (DRAM) serves as a work area for storing intermediate data produced during execution of processing.
However, the aforementioned system needs multiple memory chips, which in turn increase the packaging space or mounting area so as to increase the manufacturing cost. For this reason, it is required to further reduce the size of the system and to further downsize the system.
In order to further reduce the size of the system, it is necessary to reduce the number of memory chips included in portable small-size electronic devices.
It is possible to divide the overall storage area of a reloadable ROM into two areas, i.e., an area defined by a certain range of address for storing program codes and a work area defined by another address. This allows the system to be realized using a single memory chip.
In order to reload data after being erased from the reloadable ROM, ROM may need a relatively long write time than DRAM or SRAM, which makes it impossible to perform random access, thus making the work area unusable.
Magnetoresistive random-access memory (MRAM) may be equivalent to DRAM in terms of the write time; however, MRAM is bigger than DRAM in the chip size thereof so that it cannot be suited to memory having a large storage capacity.